The present application is related to U.S. Pat. No. 6,789,026 entitled “Circuit and method for monitoring battery state of charge”, filed May 2, 2003; U.S. Pat. No. 6,832,171 entitled “Circuit and method for determining battery impedance increase with aging”, also filed May 2, 2003; U.S. Pat. No. 6,892,150 entitled “Circuit and method for measurement of battery capacity fade”, also filed May 2, 2003; and U.S. Pat. No. 7,443,144 entitled “Method and apparatus for operating a battery to avoid damage and maximize use of battery capacity by terminating battery discharge”, filed Jan. 11, 2006. These patents are assigned to the assignee of the present application and are incorporated herein by reference for all that they teach and disclose.
Proper control of a rechargeable battery, during charging or discharging, depends on accurate estimation of a present state of the battery expressed as a state of charge (or conversely a depth of discharge—DOD) or remaining capacity or remaining usage time or other appropriate quantity. Inaccurate estimation of the state of the battery can result in damage to the battery, damage to surrounding circuitry in the battery-operated host device, injury to a user of the host device, loss of data in the host device and/or highly inefficient usage of the battery, among other potential problems.
Battery state estimation is generally the function of a battery fuel (gas) gauge circuitry in the host device or a battery pack. The typical battery fuel gauge requires a full charge and discharge cycle to update the battery discharge capacity, which rarely occurs with “real” applications, so gauging errors frequently occur. If the battery fuel gauge circuitry is inaccurate, it could either overestimate or underestimate the remaining capacity (e.g. in milliamp-hours or percent state of charge—SOC) of the battery. Providing accurate remaining capacity information throughout battery lifetime and over temperature and usage load profiles is often an underestimated challenge mainly because the battery's useable capacity is a function of its discharge rate, temperature, aging and self-discharge. In fact, it is almost impossible to develop an algorithm to accurately model the battery's self-discharge and aging effects on the capacity.
If the battery fuel gauge circuitry significantly overestimates the remaining capacity of the battery, then the battery fuel gauge circuitry may indicate that the battery has a considerable amount of remaining capacity when the battery, in fact, has no remaining capacity. In this case, the battery may continue to discharge until it no longer produces the minimum voltage necessary for the proper functioning of the host device. The host device would then shut down or stop working without warning, thereby losing (or corrupting) any data in the device's memory. Such data loss can be catastrophic to the user of the device. The prevention of data loss is, therefore, one of the purposes of the battery fuel gauge circuitry.
On the other hand, if the battery fuel gauge circuitry significantly underestimates the remaining capacity of the battery, then the battery fuel gauge circuitry may indicate zero remaining capacity when the battery actually still has a considerable amount of charge available. Nevertheless, the battery fuel gauge circuitry will cause the host device to instigate a controlled system shut-down in this case in order to prevent a loss of data, even though the risk of data loss is not in fact imminent. No damage or data loss occurs in this case, but the user of the host device is unnecessarily inconvenienced by the early shut-down of the device and may be incorrectly led to believe that the battery or the host device does not function up to expectations.
The maker of the host device may choose to incorporate a larger, higher-capacity battery in the device in order to compensate for the inaccurate battery fuel gauge circuitry, thereby ensuring a sufficiently long battery run time. However, the battery-operated host device is usually intended to be relatively small and lightweight; whereas, this solution increases the size and weight (and usually the cost) of the device. Alternatively, the maker could choose to incorporate a “premium” battery (higher capacity in a smaller size) in the host device. However, such premium batteries are relatively expensive, which is a very significant concern, since the battery already often represents a significant portion (e.g. a third) of the overall cost of the host device.
To compensate for the potential overestimation of remaining battery capacity, the maker of the host device may choose to design the battery fuel gauge circuitry to indicate zero remaining battery capacity when the battery still has significant capacity, thereby maintaining a portion of the battery capacity as a failsafe reserve. In other words, by design, the battery fuel gauge circuitry may intentionally underestimate the actual battery capacity in order to prevent an overestimation error and a catastrophic data loss or corruption. However, if the battery fuel gauge circuitry is relatively inaccurate, it is not possible to predict when it will overestimate battery capacity and when it will underestimate battery capacity. Therefore, when the inaccuracy of the battery fuel gauge circuitry causes it to underestimate the battery capacity, then the designed-in underestimation will simply exacerbate the problem, resulting in highly inefficient battery usage.
To mitigate these problems, the battery fuel gauge circuitry must be as accurate as possible. The accuracy of the battery fuel gauge circuitry generally depends on the accuracy of parameters used to estimate the state of the battery. Such parameters generally include an internal resistance (or impedance) of the battery, an open circuit voltage (OCV) of the battery and a maximum charge capacity of the battery, among other potential parameters. The relationship between these parameters and the state of the battery is circular, since, not only does the estimation of the state of the battery depend on these parameters, but these parameters depend on the actual state of the battery. For example, there is a recursive cycle wherein the internal battery resistance (or impedance) is needed to obtain the OCV, the OCV is needed to obtain the DOD (or SOC), and the DOD is needed to obtain the internal battery resistance, and so forth.
In other words, as the state of the battery changes (as a result of charging or discharging or of an idle time), the parameters change. Additionally, as the battery ages (generally determined by the number of charge and discharge cycles the battery has undergone), these parameters and the relationships between these parameters and the state of the battery further change. Therefore, it can be necessary to update the parameters periodically in order to re-estimate the state of the battery, so that the estimated point at which discharge is to be terminated (and the host device gracefully shut down) is as dose to the actual desired point. (The aforementioned related patents describe a variety of techniques and apparatuses involving updating various parameters and estimating states of batteries.) In this manner, the most efficient use of the battery is to be achieved without risking loss of data.
There is a tradeoff between the frequency of parameter updating and overall battery performance due to the fact that operation of the battery fuel gauge circuitry necessarily consumes a portion of the battery's capacity. Therefore, more frequent updates of the parameters will consume more of the battery's capacity, noticeably decreasing the battery capacity available for operation of the host device and making it appear that the battery discharges too quickly. In other words, the need for updating parameters of the battery has to be balanced against the need for a long battery discharge time.
To ensure a long battery discharge time, the battery fuel gauge circuitry generally updates the parameters as infrequently as possible. A typical result of this practice is illustrated in battery terminal voltage vs. remaining capacity (in milliamp hours) graphs 102 and 104 in FIG. 1. The graph 102 depicts an example voltage vs. true remaining capacity of a battery. The graph 104 shows an example voltage vs. estimated remaining capacity of the battery. Additionally, the point 106 at which zero capacity remains is shown. The voltage at which discharge of the battery is terminated (end-of-discharge voltage—EDV) is also shown.
The true remaining capacity graph 102 represents an example that is generally determined in a laboratory in order to ascertain the actual remaining capacity of the battery under test relative to its terminal voltage. The estimated capacity graph 104 represents an example that can be obtained from calculating remaining capacity based on values of terminal voltage, discharge current and temperature measured during operation of the battery in a host device. The estimated capacity graph 104, therefore, includes update points 108 at which the parameters used to calculate the remaining capacity are updated, as generally described above. The update points 108 generally occur at regular intervals, e.g. as defined by a percentage of the SOC (or DOD), throughout the discharge cycle. However, only the last four of the update points 108 are indicated on the graph 104.
At the middle two update points 108, the updates result in substantial corrections to the estimated remaining capacity, as can be seen by the sudden rightward horizontal slope of the graph 104 at these two points. By the time the final update point 108 (the lowest point of graph 104) is reached, the estimated capacity appears to be negative, i.e. to the left of the zero remaining capacity point 106. In other words, the battery fuel gauge circuitry in this example will pass its shutdown point before the last update, making the final update point 108 too late to prevent a premature shutdown.